The invention relates to a paper machine fabric which comprises two separate layers made of two separate yarn systems, the yarn system made up of the warp and weft yarns forming the paper side and the yarn system made up of the warp and weft yarns forming the machine side, which are arranged to form independent structures in the warp and weft directions of the fabric, and which structures are bound together by means of binder yarns.
Conventional triple layer paper machine fabrics have two separate layers, the paper-side layer and the machine-side layer, and the layers are interconnected mainly by means of a binder weft. On the paper side, the binding is done in such a manner that the binder weft serving as a binder yarn runs alternately in phase with the cross yarn and alternately in different phase with said yarn. This results in that the binder yarn does not run straight in the cross direction. Further, at the binding point on the paper side, the binder yarn remains at nearly the same level with other surface yarns. On the machine side, the binder yarn is slightly more inside the fabric. As a result of this, the binder yarn also remains twisted in the z direction.
The twists of the binder yarn in the cross and z directions cause the binder yarn and the cross and longitudinal yarns to chafe against each other. As a result of the chafing, the yarns wear initially at the binding points of the binder yarn and later when the fabric loosens as a result of the chafing, the inner structures on the paper and machine sides chafe more and more against each other. With the wear of the inside of the fabric, the binder yarn begins to leave marking patterns on the surface of the paper, because the fabric has become thinner than its original thickness on the inside of the fabric, but the binder yarn has remained in its original dimension. A long-lasting inside wear may also cause the layers to separate from each other.
Thus, the binder weft wears the fabric from the middle, on the inside. This is due to the fact that the peripheral speeds of the paper-side layer and the machine-side layer are different in a paper machine. Another reason is the filler that enters the wire in a paper machine. The filler and the binder weft wear recesses in the warp yarns and the fabric flattens. Because of this, the binder weft remains looser and causes marking, for instance. In the worst case, the layers can even separate from each other as explained above. A further problem is that the binder weft pulls the warp yarn it binds slightly inwards on the paper side. This depression causes marking. The binder weft also causes an extra yarn flow on the surface of the fabric on the paper side. At this point, the fabric is denser and water draining from the paper web cannot evenly exit through the wire, which causes marking. In conventional fabrics, the binder weft twists from the paper side of the fabric to the machine side and back. The twisting is quite sharp and because of it, the layers on the paper and machine sides cannot come close to each other, thus making the fabric thick. This is why the fabric has a large water space. A wire having the above-mentioned structure carries a lot of water with it, which may cause splashing in the paper machine. Splashing makes the paper machine structures dirty and causes defects in the paper web, at worst even holes. A large water space of a wire also causes rewetting, in which case water from the wire re-enters the paper web and causes a reduction in dry content.
A further problem with conventional triple layer wires is that the wire stretches in the paper machine. When examining the layers on the paper and machine sides separately, it can be noted that the paper-side layer stretches considerably more than the machine-side layer, which is due to the fact, for instance, that in the conventional structure, the warp density is the same on the paper side and machine side and the paper-side warp is thinner than the machine-side warp. In addition, the stretching of the paper-side layer in relation to that of the machine-side layer is increased by the denser twisting of the warps in the paper-side layer. The more the wire stretches in the machine direction, the more it also narrows in the cross direction. Due to the stretching difference between the layers, the layer tries to narrow more than the machine-side layer. Because of this, the wire may become streaked and cause profile irregularities in the paper web. A speed difference in the top and bottom wires causes wear on the paper side of the wires, which together with a heavily worn machine side causes the wire to break.
It is an object of the invention to provide a paper machine fabric, by means of which the drawbacks of prior art can be eliminated. This is achieved by the paper machine wire of the invention, which is characterized in that the yarn system forming the paper side is arranged to comprise two warp systems which are made up of top warps and additional warps, and two weft systems which are made up of top wefts and additional wefts, whereby the top wefts are arranged to bind to the top warps only and the additional wefts to the additional warps only, that the warp system made up of the top warps of the layer forming the paper side is bound together with the warp system of the structure forming the machine side by means of binder yarns by arranging the binder yarns at the paper-side binding points to press the top warps inside the fabric in such a manner that the binder yarns are at the binding point substantially below the fabric surface, and that the additional warps are, between the binding points, arranged to run between the layer forming the paper side and the layer forming the machine side.
The invention provides above all the advantage that the binder yarn twists in the cross and z directions less than before and thus does not cause inside wearing. In addition, because the binder yarn is in the z direction straighter than before, the wire can be made substantially thinner. In this connection, it should be remembered that in a paper machine, the wire is washed during the return cycle. When the pulp spray hits the wire, it is preferable for the operation of the wire that its water content is as low as possible and evenly distributed. The thin wire structure of the invention is easy to wash and the impingement drying used in modern paper machines dries such a wire structure evenly. The machine-direction stretch difference between the layers of the wire of the invention is smaller than in conventional triple layer wires. This is due to the fact, for instance, that the warp density in the paper-side layer is higher than that on the machine side, whereby the load is more evenly distributed between layers than in a conventional triple layer wire. The solution of the invention is very flexible, and the binding can be modified as appropriate for each need, it is for instance possible to use binder yarn pairs instead of a binder yarn. A further advantage is that the binder yarn remains inside the fabric, i.e. the binder yam does not come to the paper-side surface and thus does not cause marking. The fabric of the invention does not easily break, because its paper-side warps are not immediately vulnerable to paper-side wear. The paper machine fabric of the invention is also advantageous, because the high yarn density on the paper side gives the paper web a good support.